1. Field of the Invention
The present invention relates to a single-acting directional control valve which is driven by a pilot fluid pressure or the like and returned by biasing force of a return spring.
2. Description of the Related Art
A single-acting directional control valve illustrated in FIG. 4 is typically known as a component of a combination valve such as disclosed in Japanese Unexamined Patent Application Publication No. 2008-95818.
The directional control valve has a valve body 10 and a valve chest 11 formed thereinside, and the valve chest 11 formed inside the valve body 10 has openings at its two ends which are airtightly sealed by a lid 12 and a cover 13, respectively. A valve rod 30 is provided inside the valve chest 11 and driven in the direction of an axial line L, and the valve chest 11 is in communication with a supply port 19, an output port 20, and a discharge port 21 which open in the direction orthogonal to the direction of the axial line L and are aligned in sequence from one end of the valve chest 11 to the other end. Partitions 15 and 16 are provided between the supply port 19 and the output port 20 and between the output port 20 and the discharge port 21 to define the valve chest 11, respectively, and retainers 27 and 28 having a plurality of through-holes are provided between the partition 15 and the lid 12 and between the partition 16 and the cover 13 to guide the movement of the valve rod 30 in the direction of the axial line L, respectively.
The valve rod 30 which is supported by the retainers 27 and 28 so as to be able to slide inside the valve chest 11 in the direction of the axial line L has a guided portions 31 and 32 provided at its two ends in the direction of the axial line L so as to be guided by the inner surfaces of the retainers 27 and 28, respectively, and a disk-shaped poppet valve 37 provided between the guided portions 31 and 32 with narrow portions 35 and 36 interposed therebetween, respectively. The poppet valve 37 has a left side surface 37a and a right side surface 37b provided with valve seats 38a and 38b, respectively, and has a poppet-type structure, and the valve seats 38a and 38b are configured so as to face a supply valve seat 39a and a discharge valve seat 39b which are formed on the opposite end faces of the retainers 27 and 28, respectively.
The directional control valve is a single-acting type driven by a pilot fluid pressure. The lid 12, the retainer 27, and the end face of the guided portion 31 provided at an end of the valve 30 define space as a pilot pressure chamber 40, and the pilot pressure chamber 40 is in communication with a pilot fluid pressure source through a pilot valve (not illustrated); on the other hand, a return spring 41 is provided between the guided portion 32 provided at the other end of the valve rod 30 and the cover 13 to generate a biasing force to return the valve rod 30 to a return position in the case where a fluid pressure in the pilot pressure chamber 40 has been discharged.
In the single-acting directional control valve having the above-mentioned configuration, pilot air is supplied to the pilot pressure chamber 40, and then the valve rod 30 is driven by a pilot air pressure which acts on the end face of the guided portion 31 provided at an end of the valve rod 30 and then moves to the right side in FIG. 4. Then, the valve seat 38a of the poppet valve 37 separates from the supply valve seat 39a to largely open the channel therebetween, and the valve seat 38b is pressed against the discharge valve seat 39b to close the channel between the valve seat 38b and the discharge valve seat 39b, so that compressed air from the supply port 19 is output from the output port 20 to predetermined equipment.
In the case where the pilot air in the pilot pressure chamber 40 has been discharged, the biasing force of the return spring 41 moves the valve rod 30 from the switching position described above to the left side in the drawing. The valve seat 38a of the poppet valve 37 is pressed against the supply valve seat 39a to close the channel between the valve seat 38a and the supply valve seat 39a, and the valve seat 38b separates from the discharge valve seat 39b to open the channel therebetween, so that compressed air from the output port 20 is discharged from the discharge port 21 to the exterior.
In such a single-acting directional control valve, at an early stage at which the pilot air in the pilot pressure chamber 40 is discharged and the biasing force of the return spring 41 causes the valve seat 38b of the poppet valve 37 to separate from the discharge valve seat 39b, compressed air at high pressure flows from the output port 20 toward a space around the narrow portion 36 through the gap between the discharge valve seat 39b and the valve seat 38b as indicated by an arrow A. In the case where the compressed air rapidly flows toward the center of the valve rod through the gap as described above, a rapid air stream causes air around the side surface 37b of the poppet valve 37 to flow out with the result that a low-pressure area is generated in the vicinity of the side surface 37b, the side surface 37b being at the side of the discharge port 21. In the case where a decrease in the pressure is large, the poppet valve 37 is pushed back against the biasing force of the return spring 41 in some cases with the result that a switching operation of the valve rod 30 by the biasing force of the return spring 41 becomes unstable.
Such an unstable switching operation can be overcome by, for instance, an enhancement of the biasing force of the return spring 41; however, this needs approaches to be attempted, such as an enhancement of a fluid pressure of the pilot air to be supplied to the pilot pressure chamber 40 and an increase in a pressure-receiving area of the valve rod 30 in the pilot pressure chamber 40. Since such approaches have a large effect on the configuration or design of the directional control valve, it is desirable to develop a technique which can easily prevent the generation of a low-pressure area in the vicinity of the side surface 37b of the poppet valve 37, the side surface 37b being at the side of the discharge port 21.